Growth kinetics, spatialization and quality of potato tubers monitored in situ by MRI - long-term effects of water stress.

Understanding the potato tuber development and effects of drought at key stages of sensitivity on yield is crucial, particularly when considering the increasing incidence of drought due to climate change. So far, few studies addressed the time course of tuber growth in soil, mainly due to difficulties in accessing underground plant organs in a non-destructive manner. This study aims to understand the tuber growth and quality and the complex long-term effects of realistic water stress on potato tuber yield. MRI was used to monitor the growth kinetics and spatialization of individual tubers in situ and the evolution of internal defects throughout the development period. The intermittent drought applied to plants reduced tuber yield by reducing tuber growth and increasing the number of aborted tubers. The reduction in the size of tubers depended on the vertical position of the tubers in the soil, indicating water exchanges between tubers and the mother plant during leaf dehydration events. The final size of tubers was linked with the growth rate at specific developmental periods. For plants experiencing stress, this corresponded to the days following rewatering, suggesting tuber growth plasticity. All internal defects occurred in large tubers and within a short time span immediately following a period of rapid growth of perimedullary tissues, probably due to high nutrient requirements. To conclude, the non-destructive 3D imaging by MRI allowed us to quantify and better understand the kinetics and spatialization of tuber growth and the appearance of internal defects under different soil water conditions.

First report of Xanthomonas campestris pv. campestris causing black rot on oilseed rape (Brassica napus L.) in France.

In October 2022, v-shaped necrotic lesions were observed on the leaf margins of field-grown winter oilseed rape (WOSR), Brassica napus L., in western France (Ille-et-Vilaine (35) and Maine-et-Loire (49) departments). Disease incidence on volunteers and cultivated WOSR was generally low (5-10 %) but occasionally up to 80% on some fields. Leaf sections sampled from the margin of necrotic leaf tissue were dilacerated in sterile deionized water and the extract was spread onto tryptone soya agar (TSA) with cycloheximide (100 mg.L-1) and Polyflor (Syngenta, France) (2ml.L-1, containing 5 mg.L-1 propiconazole) then incubated at 28°C for 2 days. Colonies were yellow-pigmented, mucoid, and convex, which are morphological characteristics of Xanthomonas spp. colonies. The partial fyuA and gyrB gene sequences were amplified for eight isolated strains (CFBP 9155, CFBP 9156, CFBP 9157, CFBP 9158, CFBP 9159, CFBP 9161, CFBP 9162, and CFBP 9163) using primers of Fargier et al. (2011), and sequenced (Genoscreen, France). The sequences were deposited under numbers OR232891 to OR232898 for fyuA and OR634932 to OR634939 for gyrB. BLASTN analysis of the sequenced fyuA amplicon showed 100% identity and query coverage with the fyuA fragment of Xanthomonas campestris pv. campestris (Xcc) CFBP 6865R (Bellenot et al., 2022). BLASTN analysis of the sequenced gyrB amplicon showed two allelic forms: one showed 100% identity and query coverage with the gyrB fragment of Xcc strain CFBP 6865R (Bellenot et al., 2022), the other one showed 100% identity and query coverage with the type strain Xcc CFBP 5241 (ATCC33913) (Vorhölter et al., 2003). Moreover, two qPCR tools were used to identify the strains successfully as Xcc (Köhl et al., 2011; Rezki et al., 2016) which target the same gene encoding a hypothetical protein and whose primers overlap. The pathogenicity of the eight isolated strains was validated using a bacterial suspension (108 CFU.ml-1) for i) leaf spraying until runoff onto the leaf surfaces of WOSR plants previously maintained at saturated humidity for 48 hours, ii) wound-leaf inoculation of the two youngest true leaves with scissors that had been dipped into the bacterial suspension. Both tests were performed on 3-week-old WOSR plants of the Aviso (INRAE) genotype. Deionized water was used as negative control. Strains CFBP 5241 and the strain CFBP 4954 (Fargier et al., 2007) were used as positive controls for disease expression. Tested plants (seven for spray inoculation and four for wound-leaf inoculation per strain and control condition) were incubated in a greenhouse at 20°C/24°C (night/day). Isolated strains and the strain CFBP 4954 caused yellow lesions with both inoculation methods that necrotized starting about 10 days post inoculation (dpi). The spots coalesced within 14 dpi to form necrotic areas. The type strain CFBP 5241 caused mild symptoms, with only yellow lesions that did not coalesce. Plants inoculated with water remained symptomless. To complete Koch's postulate, re-isolations were achieved. Re-isolated strains on TSA showed the same colony morphology as described above. All re-isolated strains were identified as Xcc based on partial gyrB sequencing and Xcc specific qPCR test (Rezki et al., 2016). This first report in France and the recent identification in Serbia (Popovic et al., 2013) may illustrate the emergence of the disease on this crop in Europe. The prevalence and consequences of this disease should be evaluated over a wider geographic area.

Characterization of the Water Shortage Effects on Potato Tuber Tissues during Growth Using MRI Relaxometry and Biochemical Parameters.

The potato is one of the most cultivated crops worldwide, providing an important source of food. The quality of potato tubers relates to their size and dry matter composition and to the absence of physiological defects. It depends on the spatial and temporal coordination of growth and metabolic processes in the major tuber tissues: the cortex, flesh and pith. In the present study, variations in the biochemical traits of each of these tissues were investigated during tuber growth under optimal and water-deficit conditions. MRI relaxometry was used as a non-invasive and quantitative method to access information on cellular water status. The presence of slight but significant variations in organic compound contents quantified in the cortex and flesh revealed a tissue-dependent metabolic pattern. The T2 and relative I0 of the bi-exponential relaxation signal allowed a distinction to be made between the pith and the cortex, whereas the flesh could be differentiated from these tissues only through its relative I0. T2 values did not vary significantly during tuber development, in accordance with the typical growth pattern of tubers, but were shown to be sensitive to water stress. The interpretation of the multi-exponential transverse relaxation times is discussed and could be further developed via microscopic analysis.